A Quick Review
of Energy Systems
Whether it's during a 26 mile marathon run or one
explosive movement like kicking a ball, skeletal
muscle is powered by one and only one compound --
ATP (adenosine triphosphate).
But
this poses a problem. The body only stores a small
quantity of this 'energy currency' -- enough to
power just a few seconds of all-out,
explosive exercise. In order to meet your energy
demands. The
body must replace or resynthesize ATP on an
ongoing basis.
Understanding how it does this is the key to
understanding energy systems. There are 3
pathways the body uses to resynthesize ATP
called metabolic pathways. One of these
pathways will predominate depending on the
activity...
ATP-PCr System
Remember there is only a small quantity of ATP
stored in your muscles. When this runs out you have
a reservoir of a similar high energy compound called
PCr (phosphocreatine). Combined, ATP and PCr
will give you about 5 to 8 seconds worth of energy
in all-out activity... not much is it? If
activity continues after the stores of ATP and PCr
have run out, the lactic acid system begins
to predominate.
Lactic Acid System
(Anaerobic Glycolysis)
Very simply, carbohydrate stored in the body is
broken down and used to form ATP in a process known
as anaerobic glycolysis.
The
chemical reactions that take place during the lactic
acid system, do so without the presence of
oxygen. So it's labelled an anaerobic
process. And as you probably know, a by-product of
anaerobic exercise is lactic acid.
This system will power an athlete for around 45
seconds. What happens after that? The lactic acid
accumulating in your muscles will either put a
halt to your progress or the exercise intensity
declines and you can continue. Now the
aerobic system starts to predominate...
Aerobic or Oxidative System
As exercise duration extends over 2 minutes the
lactic acid system is relied on less and less. The
body gradually shifts towards an aerobic pathway
to replenish those initial ATP stores. The longer
exercise continues the more reliance is placed on
the aerobic system.
The
body can breakdown fat or carbohydrate
to create ATP within the aerobic system.
Carbohydrate is the first substrate the body uses.
The switch to fat as a source of fuel depends on
several factors such as exercise intensity, training
status and diet. As
the intensity of exercise is much lighter and the
demand for energy declines, the body has time to use
oxygen in the chemical reactions. Hence it is
labeled aerobic exercise.
Energy Systems in
Sport
Applying energy systems to some sports can get
quite complicated.
It's easiest to look at straight
forward running events first.
100m Sprint
Top athletes run this event regularly under 10
seconds. The ATP-PCr system powers a sprinter
for most of the race.
If
you watch a slow motion reply of a 100m sprinter you
will notice they don't breathe. With pursed lips
their face is a picture of concentration. All of
their energy production is from anaerobic
processes that occur without oxygen.
800m Run
Just as with the 100m, an athlete is powered by the
ATP-PCr energy system for the first few
seconds. Because the athlete is not running 'all
out' the stores of ATP and PCr will last a few
seconds longer.
The
lactic acid system predominates for the rest
of the race but the aerobic system does make a
contribution. With energy systems the emphasis is on
what predominates. No activity, whether it's
a gentle stroll or an all out sprint, uses just one
energy system exclusively... they all contribute to
some extent.
Half Marathon
Without doubt the aerobic system makes the
greatest contribution to this event. The other two
energy systems will predominate during the first
minute or so of the race and in a sprint finish.
What determines whether the athlete is 'burning'
carbohydrate or fat during the run?
Well, as the aerobic system begins to predominate,
carbohydrate (in the form of glucose and
glycogen) will make the greatest contribution to
energy production. If exercise intensity is
relatively low, fat will be relied on more and
more as the duration increases. Again the body
doesn't suddenly switch from one substrate to
another, the cross over is a gradual shift.
If
exercise intensity is relatively high the
body will continue to rely predominantly on
carbohydrate.
Multi Sprint Sports
So far the examples have been straight forward. What
about multi-sprint sports like
soccer? In
short, all 3 energy systems make a
significant contribution. A basketball player uses
the ATP-PCr to jump, throw and sprint up and
down the court. The lactic acid system is
taxed if the player has to make several back to back
sprints. And of course the aerobic system
makes a contribution for the entire duration of the
game.
Course Contributor Phil Davies
